Catalytic mechanism and heat exchange

Since several supported catalytic systems have sufficient activity for the POM reaction, the main topic of research is the stability of the catalysts. There are two main processes for the deactivation of the catalyst carbon deposition and sintering of the metal. Carbon deposition is due to the process of decomposition of CH4 and CO (reactions (3) and (4)). Two different kinds of carbon can be formed on the surface of the catalyst encapsulated carbon, which covers the metal particle and is the reason for physical-chemical deactivation and whiskers of carbon, which do not deactivate the particle directly but may produce mechanical plugging of the catalytic bed. 

Two mechanisms have been proposed for the POM reaction (i) The Combustion and Reforming Reactions mechanism (CRR). In this, the methane is combusted in the absence of oxygen in the first part of the catalytic bed, producing CO2 and H2O. Along the rest of the bed, and after total oxygen conversion, the remaining methane is converted to CO + H2 by SMR and CO2 reforming (reaction (2)). (ii) The Direct Partial Oxidation mechanism (DPO). CO + H2 is produced directly from methane by recombination of CHX and O species at the surface of the catalysts. 

Dissanayake et al.64 showed that the CRR is the working mechanism in a M/AI2O3 catalysts, obtaining almost complete conversion of methane at 

3 Equilibrium calculations for the POM reaction as a function of the temperature, at atmospheric pressure and 20 atm (solid line). Squares correspond to experimental results using nickel based catalysts. 

The highly exothermic combustion reaction at the top of the catalytic bed produces hot spots in the supported catalysts that increase the problem of deactivation by sintering, but also produce some results 

---